(CNN) - On the same day he endorsed the conservative Marco Rubio in the Florida Senate race, Americans for Tax Reform president Grover Norquist announced he is backing another Republican who isn't exactly a darling of the right - Sen. John McCain.

McCain has two challengers on his right flank in the Arizona Republican Senate primary - former congressman and talk show host J.D. Hayworth and Chris Simcox, a co-founder of the Minuteman movement.

Praising McCain for his long-standing opposition to federal earmarks and his signing of the ATR "Taxpayer Protection Pledge," Norquist said the 2008 Republican presidential nominee "consistently votes against tax increases and for spending restraint."

"John McCain, along with most of the Republicans, has been very good on not raising taxes," Norquist told Phoenix radio host Mike Broomhead on Tuesday. "But he's also been a leader on the issue that has always been important but hasn't been focused on sufficiently until this year, and that's the spending issue."

Norquist also took a swipe at Hayworth, who lost his House re-election bid in 2006 to Rep. Harry Mitchell.

"J.D. Hayworth, the challenger, needs to explain why he lost his House seat, okay?," he said. "If you can't get yourself re-elected to your House seat, what do you bring to the table on a Senate race?"

Referring to Hayworth's loss in 2006, Norquist said, "When you throw away a race like that it's very expensive to the rest of the party."

The former six-term congressman stepped down Friday as host of his conservative talk radio program on a radio station in Phoenix.

"We will formally announce at a later time, but we're moving forward to challenge John McCain," Hayworth told the Associated Press. "I think we all respect John. I think his place in history is secure. But after close to a quarter-century in Washington, it's time for him to come home."

Shiree Verdone, McCain's campaign manager, said in a statement, "Senator McCain has always taken every challenge seriously, and is confident with the help and support from the people of Arizona that he will be re-elected to the United States Senate."

Hayworth would not have been able to continue hosting a radio program and also legally run for federal office. Hayworth was ousted from Congress in the 2006 election and has hosted the radio show for the past few years.

"It has been a joy to spend weekday afternoons on KFYI. Now, another chapter begins. The call to public service remains, and again it may very well lead to pursuit of public office," Hayworth said in a statement posted on the radio station's website.